Rename 'kohana' to 'system' to conform to the Kohana filesystem layout. I'm comfortable with us not clearly drawing the distinction about the fact that it's Kohana.

system/libraries/Event_Observer.php

@@ -0,0 +1,70 @@
+<?php defined('SYSPATH') OR die('No direct access allowed.');
+/**
+ * Kohana event observer. Uses the SPL observer pattern.
+ *
+ * $Id: Event_Observer.php 3769 2008-12-15 00:48:56Z zombor $
+ *
+ * @package    Core
+ * @author     Kohana Team
+ * @copyright  (c) 2007-2008 Kohana Team
+ * @license    http://kohanaphp.com/license.html
+ */
+abstract class Event_Observer implements SplObserver {
+
+	// Calling object
+	protected $caller;
+
+	/**
+	 * Initializes a new observer and attaches the subject as the caller.
+	 *
+	 * @param   object  Event_Subject
+	 * @return  void
+	 */
+	public function __construct(SplSubject $caller)
+	{
+		// Update the caller
+		$this->update($caller);
+	}
+
+	/**
+	 * Updates the observer subject with a new caller.
+	 *
+	 * @chainable
+	 * @param   object  Event_Subject
+	 * @return  object
+	 */
+	public function update(SplSubject $caller)
+	{
+		if ( ! ($caller instanceof Event_Subject))
+			throw new Kohana_Exception('event.invalid_subject', get_class($caller), get_class($this));
+
+		// Update the caller
+		$this->caller = $caller;
+
+		return $this;
+	}
+
+	/**
+	 * Detaches this observer from the subject.
+	 *
+	 * @chainable
+	 * @return  object
+	 */
+	public function remove()
+	{
+		// Detach this observer from the caller
+		$this->caller->detach($this);
+
+		return $this;
+	}
+
+	/**
+	 * Notify the observer of a new message. This function must be defined in
+	 * all observers and must take exactly one parameter of any type.
+	 *
+	 * @param   mixed   message string, object, or array
+	 * @return  void
+	 */
+	abstract public function notify($message);
+
+} // End Event Observer